Many methods are known for forming the electrical interconnections between an integrated circuit and the supporting substrate. Tape automated bonding (TAB) is one commonly known method for forming these such electrical interconnections. A TAB tape is provided which comprises a plurality of individual long, slender inner leads attached to, and extending out from, generally wider, larger outer leads. There may be many of these inner/outer lead configurations on a single TAB tape.
An individual inner lead on the TAB tape is bonded to the integrated circuit at a bonding pad so as to form an inner lead bond. There are typically many of these inner lead bonds on a single integrated circuit. The inner lead bonds are typically formed by first depositing a gold bump, or other suitable material, on either the end of the TAB tape inner lead or on the integrated circuit itself. The integrated circuit and TAB tape inner leads, which are generally copper, are then aligned and simultaneously thermocompression gang bonded.
After bonding between the integrated circuit and the TAB tape inner leads is complete, the integrated circuit assembly is excised from the TAB tape at a point beyond the outer lead, so that the outer lead remains attached to the bonded inner lead and integrated circuit. The integrated circuit assembly is subsequently mounted on the substrate and the outer leads are appropriately bonded to the substrate.
In an alternative bonding method, a flexible circuit (FLEX) is used to provide the individual inner leads to form the electrical interconnections between the integrated circuit and the substrate, the substrate being an integral part of the flexible circuit itself. The FLEX circuit consists of a patterned arrangement of conductors on a flexible insulating base substrate with or without cover layers. The FLEX circuit may be single or double sided, multilayered or rigidized, in addition to other possible arrangements. The FLEX circuit may be formed by several methods, such as by laminating copper foil to any of several base substrate materials, or alternatively pattern plating copper directly onto the substrate.
The FLEX circuit is advantageous in that it contains both the internal and external integrated circuit chip interconnections. The inner leads are adjacent to and an integral part of the flexible circuitry pattern. Outer leads are not required, as with the TAB tape technology, because the individual inner leads are incorporated within the flexible circuitry pattern. Therefore outer lead bonds are not necessary and correspondingly the number of inner connections are substantially reduced. In addition, the flexible circuitry pattern is supported by and electrically integral with the flexible substrate at the appropriate regions.
For these reasons, FLEX circuitry technology has many advantages. FLEX circuitry significantly reduces the number of chip interconnections resulting in reduced lead inductance and lead-to-lead capacitance, as well as increased product reliability. In addition, the use of the FLEX circuitry permits smaller integrated circuits and interconnection patterns because the chip is mounted directly onto the patterned substrate.
However, despite the above-described advantages for both the TAB tape bonding and FLEX circuitry technologies, a significant disadvantage exists. Generally with these technologies, electrical current flows from the conductor, through the bond region on the integrated circuit, and subsequently proceeds through the layer of metallization on the integrated circuit to the desired region on the integrated circuit. In addition, generally separate electrical conductors are employed for each electrical interconnection formed on the integrated circuit. Therefore, if an electrical interconnection is required between discrete points on an integrated circuit, this interconnection is provided by a subsequent separate metallization layer or discrete conductive wire.
These prior art methods result in very high current densities in the bond regions and the metallization layers on the integrated circuit, particularly in high power transistor applications. In addition, because it is preferable that the thickness of the layer of metallization on the integrated circuit be minimized, it is difficult to maintain a low electrical resistance between the electrically interconnected regions on the integrated circuit.
Therefore, although there are many advantages associated with these prior art methods, wherein the interconnection leads are provided by discrete layers of metallization or conductive wires, there are disadvantages also. The prior methods are generally characterized by (1) high current density in the bond areas and the metallization layers of the integrated circuit and (2) increased electrical resistance between the interconnected regions on the integrated circuit, resulting in decreased electrical properties and performance.
It is therefore advantageous to provide an electrical interconnection lead, suitable for electrically interconnecting discrete regions of an integrated circuit, which is not formed by a separate wire or metallization layer, and which is also electrically efficient. Further, it is desirable that the provided interconnection lead be compatible with tape automated bonding (TAB) and flexible circuitry (FLEX) technologies.